1. Field of the Invention
This invention pertains generally to network communication systems, and more particularly to methods of prioritized data transmission.
2. Description of Related Art
There are two broad categories of networks: isochronous networks and non-isochronous networks. An IEEE 1394 network is a typical isochronous network in which one of the devices on the network manages time slot use. In response to requests from other devices wanting to send a stream, available time slots are allocated by a time slot manager. Once assigned, a transmitter can generally continue to use a time slot, uninterrupted by other streams, until the transmission ends. The original stream packet intervals transmitted over the network are reconstructed on the receiver side. A substantial advantage of isochronous communication is that QoS (quality of service) is completely guaranteed.
However, isochronous networks also harbor some disadvantages. For example, within a centralized network if the time slot manager is disconnected, another device has to take over the time slot management, consequently requiring that each network-centric device be configured with time slot management capability which increases network device cost. Furthermore, in response to device connection or disconnection, a full bus reset occurs that disturbs bus communications. This type of isochronous control operates satisfactorily on a dedicated noise-free network such as IEEE 1394, but it is not well suited for use on networks subject to harsh transmission conditions such as an 802.11 wireless network or a power-line network.
A typical non-isochronous network is represented by Ethernet over which traffic is often controlled using a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) mechanism, or similar. Under CSMA/CA, when a device on the bus wants to communicate with another device, the transmitter first detects a carrier on the bus. If no carrier is detected, then the device commences communication. If, however, carrier is detected then the transmitter enters a back-off mode, and after a delay reattempts the process. The wait time during backoff is generally randomized so that two waiting devices do not collide again. In conventional CSMA/CA the bus is not controlled by a time slot manager.
Utilizing a non-isochronous network provides both advantages and disadvantages. One advantage is with regard to allowing users to freely connect or disconnect devices onto, or off of, the network without the penalty of a bus reset. Each device need not be concerned with the network status, in particular what devices are connected, or are not connected to the network. The network interface is simple and low-cost because no time slot management capability is required. A disadvantage is that quality of service (QoS) is not guaranteed; when the network is busy devices are subject to indeterminate wait intervals.
A number of market forces are driving advances in networking. One such force is the expanding home networking market, which has been adopting 802.11 wireless standards and the HomePlug™ 1.0 power line network standards. Both of these standards are Ethernet-equivalent and do not guarantee quality of server (QoS). Although network standards such as IEEE 1394 guarantee QoS, they are not well suited to a number of markets due to a limited cable transmission range and because cables must be routed from device to device, such as between rooms in a household.
Accordingly, a need exists for autonomous network communication standards which provide flexibility and low cost, while being capable of supporting a given quality of service (QoS) for at least a portion of the transmissions communicated over the network. The present invention is an autonomous network that satisfies those needs, as well as others, and overcomes the deficiencies of previously developed network solutions.